Echo suppressor



.June14, 1949.-P Ic. GlLLlNGs ETAL ECHO SUPPRES SOR 2 Sheets-Sheet 1 Filed Sept. 15, v1943 I lnlnlllll Patented June 14, 1949 ECHOfl SUPPRESSGR y Charles Gillings, k Liverpool; andjPercyEdward Alexander'Cowley Coventry; England, assignors i to AutomaticTelephone Electric Company Limitedg- LiverpoolgEngland; Ia British companlyi Application-September 15, 1943, Serial-.502,5f8 In Great Britain November 14; 1942" The.' present' invention relates' to echor sup'lc'rre's'fv sors for use in teleconrimunicationJ systems; and is` more particularly concerned with" echo s'up'r pressors' of the differential type which utilise" variable attenuation networks lconnected in the go`and return circuits respectively.

The invention is moreover particularly conf c'erned with differential terminal echo 'SuppressorsinV which it is known that lock-up troubleV can' occur if an' outgoing signa1 is received over the Z-Wire terminating circuit either prior to cessation of a signal incoming over the returnl cir" cuit or during the hangover period of the echo suppressorafter it has been biassed inthe re-v turn direction. At th'e`4-wire/2-Wire junction hybrid' transformer in the echo suppressor the outgoing signal divides,v part going into the Ig-o` network where it is Arequired and part into .the incoming circuit from the return'path. Under the above conditions that partI of the outgoing signal'- which feeds into the incoming 'circuit may be suicient' .to maintain'the effect of the incorniing signal after this has'ceased so'thatthe out-V goingsignal will `continue tol block its own go circuit for as long as it persists the resultant lock-up effect being particularly serious under continuous tone signalling conditions. It isr therefore` the` chief object' of the invention t0 I provide a simple and efficient means -for over-A coming the above-mentionedtroubleby comparatively slight alterations to the circuit.

According to the invention, in .a terminatingI differential echo suppressorof the type provided with variable attenuation networks in the goand returnfpaths and arrangements for varying the attenuation of' each network under the/control ofsignal currents in theopposite' path in'whicha signal incoming over the' two-wireline at a time when the echo suppressor is lciassed in-favor of anincoming signal'over thev return path ofthe four wire line is prevented froml maintaining this bias against itselfI by deriving the control currentf'or producing` increased attenuation in` thego direction fromf'the balancingiresistance of avhybrid transformer in circuit with the return.' path'arnplier.

The invention willi be. betterl understood froml the following description of two methodsfofcarry-iri'g it into effect', referencef'being h'ad to the accompanying drawings compri'singfFigs. 1 and.2.l Both of` these solutions to the lock-upproblem,` are assumed tobe applied? to a knownl formloff diierential: terminal echar-suppressor: innvvhichi theivariable':attenuation: networks are' ofthe type1 employing:dryrectiersiA fromthiscircuit-to produce high loss-in the ,go!\` circuit; at the'same-tim'e theextension ofy si'gn'all'I energy from theI return circuit to'- this rectifier must notbeprevented.

In Fig. I the above-menrtioned 1oek=uptroub1e is overcome' by arranging' for connection to the" input'sideof the "gomamplf'er signalrectiierf .to be taken from thev-terminatinghybrid coil? bal# ance resistance, which will receive' very'littl'eeniergy 'when signalsare received overk thefZ-wire line-with the result-that yonly a small amount of," energy will beA communicated toithe gowamplilv fier' signal' rectifier: On? the'. other hand, when a; signal is communicated tothehybr'idfcoil freniv .the returnA circuit the'signal energy i's-fdividedlr between* the balance resistance and= the 2wire2 linesofthat thesignal' recti'er comesinto opera-- tion to block the' go circuit.

Fig. la shows-schematically ythe'vbridge' network' inthe echo suppressor' from which theI control currentsA for 'the' Variable .attenuationri'e-tworks` in the go and' return circuitsA arederived;v

Fig. 2 shows 'the' echo'suppressor circuit 'infparw tial schematicvformand modified-by the -fuselof an additional hybrid transformer' for connect-v ingthe-return circuitamplifer output tothel .terminatinghybridtransformer-'and also to the". input vcircuit of the fgoi amplier sig-'nali recti` e'r:

Referring nowto Fig; '1; the go and return circuits are Adesignated' GO' and RN' respective-- lyg' Whilel the" 2'wiie line designated? /RVAN .and GVANare-the variable attenuation networks" in'th'e return 'andi go circuitsv respectively, while RA and GA represent the"re-- turn andl go circuit amplifiers'. The corre=- spending-signal4 rectifersare designated '-SRRA and'SRG-A; While H "is thai-Wire/Z-wire' terIihating hybridtransformer' representedV by' its equivalenti circuit.

With the'- `echo suppressor inl av quiescent state;A av small control 'current' is'applied'to'each attenuf ation network from the `D. G. bridge inthe direc# tions-indicated bythe arrows'k iniFig. la; thiscu'r# rentl finding a circuit-'toy earth through the com#v mon' high resistance *YCI Under' this conditioni the line"-se'ries` -rectiers in each attenuation' network" are biassed` by the direct current flow-ing.` therethrough "scr :thatfboth networks are -of low attenuatom Assumenowvltiiat@ an incoming* A: f 'signalf received over the return" circuit RN. This signal extends through network RVAN and is then amplified by amplifier RA and communicated to terminals aa of the hybrid transformer H. On this hybrid transformer the balance resistance YD in conjunction with the parallel-connected impedance of the input circuit to the go amplier signal rectifier SRGA is arranged approximately to match the impedance of the line ZWL when looking into the hybrid side of the transformer LT so that the amplified signal voltage appearing across terminals aa will, in accordance with the well-known properties of the hybrid transformer, appear across terminals ff and ee but very little potential will appear across terminals dd' so that there will be practically no tendency to self-oscillation. That part of the amplified signal which appears across terminals ec', is communicated to the line ZWL while the part appearing across resistance YD is communicated to the signal rectier SRGA which in response thereto lowers the negative bias potential on the grid of the valve in the go amplifier GA. The effect is that this valve unbalances the bridge circuit of Fig. la with the result that an out-o-balance current will flow across the bridge through the networks RVAN and GVAN in the direction bottom to top in this igure. Network RVAN therefore remains at low loss while network GVAN will be of high loss due to the reversal of current iiow therethrough. In this manner the echo-suppressor is biased in favour of the return circuit.

In the case of a signal received over the line ZWL and with the echo suppressor in quiescent condition, the signal energy is divided between dd and aa on the hybrid transformer H and very little alternating voltage appears across resistance YD. The alternating voltage appearing across dd is communicated via network GVAN and thence via the go amplifier GA and the output transformer OT to the go line, a tertiary winding on this transformer communicating some of the energy into the return circuit amplier signal rectifier SRRA. The bias on the grid of the valve in the amplifier RA is thereupon decreased, whereupon the bridge network is unbalanced in the opposite direction to that obtaining with a return signal so that the resultant control current is in such a direction as to render network GVAN of low loss and network RVAN of high loss. As regards the alternating voltage appearing across aa of the transformer H, this causes an alternating signal current to be superimposed on the anode direct current ow of the valve in amplifier RA, but this is without effect as far as the RN line is concerned.

Considering now the case where the echo suppressor is biased in the return direction and a signal is received over the 2-wire line ZWL. When this signal is received there will be already a high loss in network GVAN so that it cannot be communicated to the go network. When the incoming signal over the return circuit ceases, the energy communicated to terminals aa of the transformer H and from there to the terminals ff will cease so that the bias on valve GA will restore to its normal value. The D. C. bridge network will then restore to a state of balance and both attenuation networks RVAN and GVAN will again become of low loss. The outgoing signal energy received over the 2-wire line will now take effect to bias the echo suppressor in the go direction as already described, it being appreciated that since very little of this energy can be communicated to terminals ff' of transformer H and thence to the signal rectifier SRGA, it cannot possibly maintain the block on its own go circuit and so cause lock-up.

The success of the arrangement described depends ultimately on the correct balance of the anode circuit impedance of the amplifier RA with the impedance obtaining between the points dd'. and since this balance can be readily adjusted, the scheme is extremely attractive, as carrying it into effect involves no additional components.

Referring now to Fig. 2 in which an additional hybrid transformer SH is provided to suppress the iiow of energy from the 2-wire line 2WL into its own signal rectifier SRGA, a signal received over the return circuit RN will produce an alternating voltage across terminals aa of the suppressor hybrid SH, the signal energy then being divided in usual manner between the balance resistance YE together with the input circuit to the go amplifier signal rectifier SRGA and the output terminals ee' which connect with the input terminals aa of the terminating hybrid transformer H. From terminals aa of transformer H the signal energy is communicated to the line ZWL, while the energy delivered to the input of SRGA functions to bias the echo suppressor in favour of the return circuit in the manner already described for Fig. 1.

As regards a signal received over the line 2WL, part of this signal energy is delivered to the go" network GVAN and part to the terminals aa of transformer H in usual manner. That part which extends to the go network is extended to line via the go amplifier and the suppressor is biased in the go direction by virtue of the energy derived from the tertiary winding of the go amplifier output transformer. As regards the energy delivered to terminals aa of transformer H, it is impossible for sufficient to be communicated back into the go amplifier signal rectifier SRGA to block its own circuit due to the fact that when its arrives at the suppressor hybrid transformer SH, the applied alternating voltage will appear only across terminals aa' and dd in usual manner and only a negligible amount will be delivered to the balance resistance YE and associated connection to the input of SRGA. For this reason it is impossible for energy received over the 2wire line to maintain the effect of a return" signal which has just ceased, and so t0 block its own go circuit.

The correct functioning of this arrangement depends on maintaining a correct impedance balance of the anode circuit of the RA amplifier valve with the resistance YF, which balance can be accurately adjusted so that for potentials applied to terminals ee in response to signals over the 2-wire line practically no voltage will appear across resistance YE and the connection to the input of SRGA.

It will be understood that the invention is not limited in application to the precise differential terminal echo suppressor circuit shown and described and that modifications or improvements may be made to this circuit without exceeding the scope of the present invention.

We claim:

1. For use in telecommunication systems a terminating differential echo suppressor comprising go and return paths, a variable attenuation net.. work in each of the paths, a signal rectifier associated with each path for varying the attenuation of each network under the control of signal currents in the opposite path, a two-Wire line, a

four wire/two wire hybrid transformer transferring signal currents from the return path to said two-wire line, means for deriving a signal current at one point in said circuit responsive to signal currents incoming over said two wire line, means for feeding said incoming signal currents to the go path, a balancing resistance associated with the hybrid transformer, and means connecting the input circuit of the signal rectier of the go path across said balancing resistance to derive a control current from said balancing resistance to produce increased attenuation in the go path so as to prevent a signal incoming over the two wire line at a time when the echo suppressor is biassed in favor of an incoming signal over the return path from maintaining this bias against itself.

2. For use in telecommunication systems, a terminating diiferential echo suppressor comprising go and return paths, a variable attenuation network in each of said paths, a signal rectilier associated with each path for varying the attenuation of each network under the control of signal currents in the opposite path, a two-Wire line, a four-wire/two-wire hybrid transformer for transferring signal currents incoming over said two-wire line to said go path and for transferring signal currents outgoing from said return path to said two-wire line, a balancing resistance associated with said hybrid transformer and means connecting the input circuit of the signal rectifier of the go path across said balancing resistance to derive a control current from said balancing resistance to produce increased attenuation in the go path so as to prevent a signal incoming over the two-Wire line at a time when the echo suppressor is biased in favor of an incoming signal over the return path from maintaining this bias against itself.

3. For use in telecommunication systems, a terminating diiferential echo suppressor comprising go and return paths, a variable attenuation network in each of the paths, a signal rectier associated with each path for varying the attenuation of each network under the control of signal currents in the opposite path, a two-wire line, a iirst four-wire/tWo-wire hybrid transformer for transferring signal currents incoming over said two-wire line to said go path and for transferring signal currents outgoing from said return path to said two-wire line, a second fourwire/two-wire hybrid transformer connected between said return path and said first hybrid transformer, a balancing resistance associated with said second hybrid transformer and means connecting the input circuit of the signal rectier of the go path across said ybalancing resistance to derive a control current from said balancing resistance to produce increased attenuation in the go path so as to prevent a signal incoming over the two-Wire line at a time when the echo suppressor is biased in favor of an incoming signal over the return path from maintaining this bias against itself.

CHARLES GILLINGS. PERCY EDWARD ALEXANDER COWLEY.

REFERENCES CITED The following references are of record in the .die of this patent:

UNITED STATES PATENTS Number Name Date 1,986,516 Mitchell Jan. 1, 1935 1,992,774 Ryall Feb. 26, 1935 2,034,703 Metzger Mar. 24, 1936 2,082,357 Ryall June 1, 1937 FOREIGN PATENTS Number Country Date 493,327 Great Britain 1 Oct. 6, 1938 

